Bottom Line:
The drivers of interspecific variation in escape strategy are poorly known.The direction of the effect of these traits was consistent with selection for a reduction of monitoring costs.Thus brain size, and the cognitive functions associated with it, constitute a general framework for explaining the effects of body size, habitat structure and sociality identified as determinants of avian escape strategy.

ABSTRACTAfter detecting an approaching predator, animals make a decision when to flee. Prey will initiate flight soon after detecting a predator so as to minimize attentional costs related to on-going monitoring of the whereabouts of the predator. Such costs may compete with foraging and other maintenance activities and hence be larger than the costs of immediate flight. The drivers of interspecific variation in escape strategy are poorly known. Here we investigated the morphological, life history and natural history traits that correlate with variation in avian escape strategy across a sample of 96 species of birds. Brain mass, body size, habitat structure and group size were the main predictors of escape strategy. The direction of the effect of these traits was consistent with selection for a reduction of monitoring costs. Therefore, attentional costs depend on relative brain size, which determines the ability to monitor the whereabouts of potential predators and the difficulty of this task as reflected by habitat and social complexity. Thus brain size, and the cognitive functions associated with it, constitute a general framework for explaining the effects of body size, habitat structure and sociality identified as determinants of avian escape strategy.

f1: Effects of (a) brain mass, (b) body mass, (c) habitat openness, and (d) group size on interspecific escape strategy of birds.Escape strategy quantified by the phi index (Φ), an effect size metric that measures how immediately prey escape from predators upon detection. Larger Φ-values imply that prey escape at a distance close to the detection distance. Corrected (a) brain mass and corrected (b) body mass are residual values of these variables after controlling for their shared effect and different sizes of points reflect differences in a species’ sample size. Plots c and d show mean ± 95% confidence intervals; the number of species tested at each level is shown.

Mentions:
The minimum model retained four variables (R2 = 0.44), all with intermediate to large effect sizes: brain mass, body mass, habitat openness, and group size (full and minimum adequate models are presented in Table 1). The most important predictor of escape strategy was brain mass, with larger-brained species (after controlling for body mass) delaying escape from predators (Table 1, Fig. 1). In contrast, larger species flushed earlier from predators than smaller species (Table 1, Fig. 1). On average, species inhabiting closed habitats flushed earlier than species in open habitats (Table 1, Fig. 1). Finally, species allowed closer approach of predators as flock size increased (Table 1, Fig. 1). Our findings were robust to the use of alternative data and analyses (Supplementary Tables S1–S5).

f1: Effects of (a) brain mass, (b) body mass, (c) habitat openness, and (d) group size on interspecific escape strategy of birds.Escape strategy quantified by the phi index (Φ), an effect size metric that measures how immediately prey escape from predators upon detection. Larger Φ-values imply that prey escape at a distance close to the detection distance. Corrected (a) brain mass and corrected (b) body mass are residual values of these variables after controlling for their shared effect and different sizes of points reflect differences in a species’ sample size. Plots c and d show mean ± 95% confidence intervals; the number of species tested at each level is shown.

Mentions:
The minimum model retained four variables (R2 = 0.44), all with intermediate to large effect sizes: brain mass, body mass, habitat openness, and group size (full and minimum adequate models are presented in Table 1). The most important predictor of escape strategy was brain mass, with larger-brained species (after controlling for body mass) delaying escape from predators (Table 1, Fig. 1). In contrast, larger species flushed earlier from predators than smaller species (Table 1, Fig. 1). On average, species inhabiting closed habitats flushed earlier than species in open habitats (Table 1, Fig. 1). Finally, species allowed closer approach of predators as flock size increased (Table 1, Fig. 1). Our findings were robust to the use of alternative data and analyses (Supplementary Tables S1–S5).

Bottom Line:
The drivers of interspecific variation in escape strategy are poorly known.The direction of the effect of these traits was consistent with selection for a reduction of monitoring costs.Thus brain size, and the cognitive functions associated with it, constitute a general framework for explaining the effects of body size, habitat structure and sociality identified as determinants of avian escape strategy.

ABSTRACTAfter detecting an approaching predator, animals make a decision when to flee. Prey will initiate flight soon after detecting a predator so as to minimize attentional costs related to on-going monitoring of the whereabouts of the predator. Such costs may compete with foraging and other maintenance activities and hence be larger than the costs of immediate flight. The drivers of interspecific variation in escape strategy are poorly known. Here we investigated the morphological, life history and natural history traits that correlate with variation in avian escape strategy across a sample of 96 species of birds. Brain mass, body size, habitat structure and group size were the main predictors of escape strategy. The direction of the effect of these traits was consistent with selection for a reduction of monitoring costs. Therefore, attentional costs depend on relative brain size, which determines the ability to monitor the whereabouts of potential predators and the difficulty of this task as reflected by habitat and social complexity. Thus brain size, and the cognitive functions associated with it, constitute a general framework for explaining the effects of body size, habitat structure and sociality identified as determinants of avian escape strategy.